Remove limits of ES6 features in standard library code

ChangeLog.md

@@ -27,6 +27,10 @@ See docs/process.md for more on how version tagging works.
   `warning: treating 'c' input as 'c++' when in C++ mode`.  This also means that
   the `DEFAULT_TO_CXX` setting now only applies when linking and not when
   compiling. (#20712)
+- JavaScript library code can now use the full range ES6 features and we rely
+  on closure compiler to transpile for ES5 when targetting older browsers.
+  For those that want to would rather perform transpilation seperately outside
+  of emscripten you can use the `-sPOLYFILL=0` setting.  (#20700)
 
 3.1.49 - 11/14/23
 -----------------

emcc.py

@@ -2318,7 +2318,7 @@ def phase_linker_setup(options, state, newargs):
 
   setup_environment_settings()
 
-  if options.use_closure_compiler != 0:
+  if options.use_closure_compiler != 0 and settings.POLYFILL:
     # Emscripten requires certain ES6 constructs by default in library code
     # - https://caniuse.com/let              : EDGE:12 FF:44 CHROME:49 SAFARI:11
     # - https://caniuse.com/const            : EDGE:12 FF:36 CHROME:49 SAFARI:11

test/test_other.py

@@ -12990,7 +12990,7 @@ def test_hello_function(self):
   })
   @crossplatform
   def test_es5_transpile(self, args):
-    self.emcc_args += args
+    self.emcc_args += ['-Wno-transpile'] + args
 
     # Create a library file that uses the following ES6 features
     # - let/const
@@ -13006,6 +13006,11 @@ def test_es5_transpile(self, args):
         let obj = Object.assign({}, {prop:1});
         err('prop: ' + obj.prop);
 
+        // for .. of
+        for (var elem of [42, 43]) {
+          err('array elem: ' + elem);
+        }
+
         // arrow funcs + const
         const bar = () => 2;
         err('bar: ' + bar());
@@ -13015,14 +13020,14 @@ def test_es5_transpile(self, args):
         var obj2 = {
           [key]: 42,
         };
-        err('value: ' + obj2[key]);
+        err('computed prop: ' + obj2[key]);
 
         // Method syntax
         var obj3 = {
           myMethod() { return 43 },
         };
         global['foo'] = obj3;
-        err('value2: ' + obj3.myMethod());
+        err('myMethod: ' + obj3.myMethod());
 
         // Nullish coalescing
         var definitely = global['maybe'] ?? {};
@@ -13042,6 +13047,15 @@ def test_es5_transpile(self, args):
       }
     });
     ''')
+    expected = '''\
+prop: 1
+array elem: 42
+array elem: 43
+bar: 2
+computed prop: 42
+myMethod: 43
+'''
+
     create_file('test.c', 'extern void foo(); int main() { foo(); }')
     self.emcc_args += ['--js-library', 'es6_library.js']
     self.uses_es6 = True
@@ -13073,26 +13087,26 @@ def check_for_es6(filename, expect):
     # Check that under normal circumstances none of these features get
     # removed / transpiled.
     print('base case')
-    self.do_runf('test.c', 'prop: 1\nbar: 2\n')
+    self.do_runf('test.c', expected)
     check_for_es6('test.js', True)
 
     # If we select and older browser than closure will kick in by default
     # to transpile.
     print('with old browser')
     self.emcc_args.remove('-Werror')
     self.set_setting('MIN_CHROME_VERSION', '10')
-    self.do_runf('test.c', 'prop: 1\nbar: 2\n', output_basename='test_old')
+    self.do_runf('test.c', expected, output_basename='test_old')
     check_for_es6('test_old.js', False)
 
     # If we add `--closure=0` that transpiler (closure) is not run at all
     print('with old browser + --closure=0')
-    self.do_runf('test.c', 'prop: 1\nbar: 2\n', emcc_args=['--closure=0'], output_basename='test_no_closure')
+    self.do_runf('test.c', expected, emcc_args=['--closure=0'], output_basename='test_no_closure')
     check_for_es6('test_no_closure.js', True)
 
     # If we use `--closure=1` closure will run in full optimization mode
     # and also transpile to ES5
     print('with old browser + --closure=1')
-    self.do_runf('test.c', 'prop: 1\nbar: 2\n', emcc_args=['--closure=1'], output_basename='test_closure')
+    self.do_runf('test.c', expected, emcc_args=['--closure=1'], output_basename='test_closure')
     check_for_es6('test_closure.js', False)
 
   def test_gmtime_noleak(self):

tools/building.py

@@ -513,7 +513,8 @@ def closure_transpile(filename):
   user_args = []
   closure_cmd, env = get_closure_compiler_and_env(user_args)
   closure_cmd += ['--language_out', 'ES5']
-  closure_cmd += ['--compilation_level', 'WHITESPACE_ONLY']
+  closure_cmd += ['--compilation_level', 'SIMPLE_OPTIMIZATIONS']
+  closure_cmd += ['--formatting', 'PRETTY_PRINT']
   return run_closure_cmd(closure_cmd, filename, env)